i have the same thing at the moment, it has happened twice in as many weeks.
the first time i gained over 300 new contacts and the second time over 200.
the other strange thing is that there are no emails in my sent folder, even though i know people are recieing emails from my address? i am also getting delivery failure notices though..
after the first instance i changed my password and had only been accessing my inbox via my phone, so i don't see it can have happened again unless there is a problem at hotmails end?